The present invention relates to a device for generating seismic impulses inside a hole or well drilled in the earth, and more particularly, to a device for generating impulses through an impact of a weight onto a target element coupled with the wall of the hole or drilling well, especially in the field of terrestial seismic prospecting.
Several devices has been proposed for applying acoustic impulses to the walls of a well, and, for example, a so-called "gas gun" type has been proposed which includes a tubular body equipped with openings communicating with means for intermittently releasing a highly-pressurized gas, which gas may be compressed air or a combustion product of an explosive mixture. The acoustic impulses are transmitted to the walls of the well through water or the sludge contained therein. Devices of this type are described in, for example, French Pat. No. 2,311,322 or the U.S. Pat. No. 3,380,551.
It is also possible, as proposed in U.S. Pat. No. 4,207,961, to generate seismic impulses by a sudden ejection of pressurized water inside a well filled with water.
Of the devices proposed for applying seismic shocks to the walls of a well by means of impacts, such devices may comprise one or several elements, portable with regard to a tubular body lowered into the drilling on the end of a cable and to which pertinent motor devices imparts significant radial accelerations and which then arrive and strike the wall of the well, thus generating impulses which are transmitted to the surrounding formations. These motor devices may, for example, be pistons connected to the portable elements and driven inside combustion chambers by the explosion of explosive mixtures. As proposed, for example, in French Pat. No. 2,492,111.
Moreover, devices have been proposed which include coupling parts connected to a body, which are deflected laterally in relation to the latter in such a way as to strike against or sink into the walls. Portable mobile weights inside the body are connected to pertinent motor devices which impart rapid movements to them. The impact forces when mobile weights collide or strike a solid wall of the body are then mechanically transmitted to the formations traversed by the drilling by the coupling parts in a manner described, for example, in French Pat. No. 2,442,455.
Drawbacks or disadvantages of the above-mentioned types of impact devices reside in the fact that the shocks, applied to geological formations traversed by the well have a fairly low amplitude since the mobile parts are relatively small and light and have a relatively short travel path or the motor devices which can be accommodated inside a cylindrical body, very frequently of short diameter, generally have a limited output power.
According to the invention, the device for generating acoustic impulses inside a well or drilling hole includes a target element, a weight for striking the target element, means for guiding the fall of the weight onto the target element and a lifting system for raising the weight in relation to the target element. A coupling element is provided which includes a variable volume enclosure adapted to be applied by volume expansion against the side wall, with the target element being fixed to the coupling element, and means are also provided for intermittently inflating the enclosure, with the inflation means including, for example, a hydraulic circuit and a pressurized water supply system.
The present invention offers the advantage that the sound energy generated at the moment of impact is considerable due to the fact that the height of the column rising above the target element, and, consequently, the kinetic energy capable of being applied to this element, may be very considerable also. This sound energy is particularly effectively transmitted to the surrounding formations due to the tight coupling existing between the inflatable enclosure supporting the target element and the walls of the well.
Moreover, with sound energy applied to the earth at a certain depth, preferably beneath the surface bed or altered bed, it is possible to avoid multiple reflections over this bed of seismic waves which disturb the recordings made from surface seismic sources.
Other characteristics and advantages of the device according to the invention will appear upon reading the description of a preferred yet non-restrictive realization mode and by referring to the attached drawings .